1. Field of the Invention
This invention relates generally to laser amplifiers and more particularly to high average power cw laser amplifiers.
2. Description of Related Art
Laser amplifiers have been built using neodymium doped vanadate crystals as the gain media. An end-pumped amplifier has been demonstrated using a fiber coupled diode bar as the pump source. A mode-locked oscillator that produces 1.8 W of average power has been amplified to an average power of 6 W pumped by two cw fiber-coupled diode bars, each producing 13 W of pump power. The gain medium was an 8 mm vanadate crystal with a doping of 0.5%. For this application, vanadate was found to be superior to Nd:YAG or Nd:YLF in part because of vanadate""s higher emission cross section. This was described in J. D. Kafka, et al., J. Opt. Soc. Am. B., 12, page 2147 (1995). Kafka and Pieterse disclose one configuration of this amplifier in U.S. Pat. No. 5,812,308.
Systems have been built with multiple stages of vanadate amplifiers. An example of such a system amplified a mode locked vanadate oscillator that produced 4.2 W of average power. The output of the oscillator was amplified to 42 W in one double-pass and three single-pass vanadate amplifiers. Each of the amplifiers was pumped by two cw fiber-coupled diode bars, which generated a total of 25 W of diode power for each stage. This system was described in A. Nebel, et al., CLEO 1998, OSA Technical Digest, postdeadline paper CPD3. The high gain observed in these systems was due to a combination of the high emission cross-section of vanadate and use of an end-pumping geometry.
Diode bars have traditionally been limited to about 40 W per bar at 808 nm. To achieve higher pump powers, stacks of diode bars have been made as two-dimensional arrays. One example is part number MMO-808-240-03, commercially available from Opto-Power Corporation, Tucson, Ariz., a stack of six diode bars, each producing 35 W of pump power for a total of 200 W. The output area of the stack is typically 1 cm by 1 cm. The pump power needs to be concentrated in a smaller spot in order to obtain high gain from an end pumped vanadate amplifier.
Stacks of diode bars have been used to pump high power oscillators as described in C. Bibeau, et al., OSA TOPS Vol. 10 Advanced Solid State Lasers, 1997, page 276. In this paper, Yb:YAG was selected as the gain medium because it has a low quantum defect. The low quantum defect resulted in less heat generation during lasing than comparable Nd-based laser systems. For systems that use high average power cw stacks, care must be taken to avoid thermal effects. Thus Yb:YAG has been the material of choice.
Pulsed diode lasers have also been used in a stack pumped oscillator. In this case, although the peak power is high, the average power is much lower and thermal problems are much less important. The combination of vanadate and a pulsed diode stack has been demonstrated. Vanadate was selected because of its higher absorption coefficient which maintains a gain deposition in a small volume. The pulsed diode lasers were limited to only 2 W of average power and the vanadate crystal was intentionally highly doped at 1% to maintain the gain deposition in a small volume. This system is described in G. Feugnet, et al., SPIE Proceeding Vol. 2698, page 105, 1996. This teaches a system that is not applicable for high average power systems. A reduced gain volume is not desirable in a high power cw pumped system since it would lead to strong thermal effects. There is a need for a high gain, cw amplifier system to achieve output powers greater than 40 W. There is a further need for an amplifier system that provides for extraction of at least 20 W from a single stage.
Accordingly, it is an object of the present invention to provide a high gain, cw amplifier system to achieve output powers greater than 40 W.
Another object of the present invention is to provide an amplifier system that provides for extraction of at least 20 W from a single stage.
Yet another object of the present invention is to provide a method of amplifying an input beam and produce output powers of at least 40 W.
These and other objects of the present invention are achieved in an amplifier with a two-dimensional array of cw diodes that produce a pump beam. A coupler is positioned to receive the pump beam. The coupler reduces a cross-sectional dimension of the pump beam and creates a modified pump beam. A vanadate gain medium is positioned adjacent to the coupler. The vanadate gain medium absorbs at least a portion of the modified pump beam and is positioned to receive an input beam from an input beam source.
In another embodiment of the present invention, a method of amplifying an input beam includes providing an amplifier that has a two-dimensional array of cw diodes that produces a pump beam. A coupler is positioned to receive the pump beam and create a modified pump beam. The amplifier has a vanadate gain medium. An input beam source is provided which produces the input beam. The modified pump beam pumps the vanadate gain medium. The input beam is directed to the vanadate gain medium, which then produces an amplified input beam.